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I've been thinking about this for quite some time. In fact, the ability
to subclass

weakref.ref objects that appeared in Python 2.4 anticipates this. :)

I like the proposal but I don't understand some details:

- Why do you want to use '__setitem__' method instead of for example
'notify'? Is it only for "make this go fast"?
- Why are such values selected for the state constants (persistent.GhostState
= None, persistent.ChangedState = True, persistent.SavedState = False,
persistent.ReadState = False)?